The present invention relates to a system for controlling a switching frequency in a switching power source apparatus of a ringing choke converter system.
Generally, a switching power source apparatus of a ringing choke converter system includes a switching transistor. Self-excitation is caused by repeatedly turning on and off this transistor. Upon self-excitation, a current having a sawtooth wave is generated at the output of the switching transistor. The current is smoothed by a capacitor, and continuously supplied to a load as a DC current. In this case, when the switching frequency of the switching transistor is kept constant, an output voltage varies depending on the magnitude of a load to be connected. More specifically, when the load is small, the output voltage from the power source apparatus becomes higher than a reference voltage, whereas when it is large, the output voltage becomes lower than the reference voltage. For this reason, the switching transistor is controlled such that when the load is small, the switching frequency is kept high, and when it is large, the switching frequency is kept low.
In conventional systems, the frequency can be controlled to be set at a low level. However, problems have been posed in control for setting the frequency at a high level. More specifically, since the maximum frequency is solely determined by the characteristics of a switching transistor, control for setting the output voltage from the power source to be constant by increasing the switching frequency of the switching transistor is greatly limited. For this reason, in the conventional systems, a dummy resistor is connected in parallel to the output terminal of the power source apparatus so as to increase the magnitude of the load Output dummy resistor loss P is represented by the following equation. EQU Output dummy resistor loss P =(Vi.sup.2 .times.Ton.sup.2)/(2.times.L.times.T) (1)
where
Vi: input voltage PA0 L: primary inductance of transformer PA0 T: switching frequency PA0 Ton: minimum ON time of switching transistor
In the conventional systems, the ratio of the ON time to the OFF time (duty ratio) of a transistor is 1:1. Therefore, as is apparent from equation (1), if frequency T is raised, output dummy resistor loss P is increased, resulting in a decrease in output voltage. When the output energy is, e.g., 5 W, a 2.5-W resistor is used as such an output dummy resistor. As a result, efficiency of the power source is degraded. In addition, a cooling fan is required because heat is generated due to the dummy resistor loss.